Monday, November 26, 2018

Flying More Efficiently

I have another new working paper out, coauthored with Zsuzsanna Csereklyei on airline fleet fuel economy. Zsuzsanna worked as research fellow here at the Crawford School on my Australian Research Council funded DP16 project on energy efficiency and the rebound effect. This paper reports on some of our research in the project. We also looked at energy efficiency in electric power generation in the US.

The nice thing about this paper is that we have plane level data on the aircraft in service in 1267 airlines in 174 countries. This data is from the World Airliner Census from Flight Global. We then estimated the fuel economy of 143 aircraft types using a variety of data sources. We assumed that the plane would fly its stated range with the maximum number of passengers and use all its fuel capacity. This gives us litres of fuel per passenger kilometre. Of course, many flights are shorter or are not full, and so actual fuel consumption per passenger kilometre will vary a lot, but this gives us a technical metric which we can use to compare models.

The graph shows that the fuel economy of new aircraft has steadily improved over time. One of the reasons for the scatter around the trendline is that large aircraft with longer ranges tend to have better fuel economy than small aircraft:

This is also one of the reasons why fuel economy has improved over time. Still, adjusted for size, aircraft introduced in earlier decades had (statistically) significantly worse fuel economy than more recent models. We used these regressions to compute age and size adjusted measures of fuel economy, which we used in our main econometric analysis.

The main analysis assumes that airlines choose the level of fuel economy that minimizes costs given input prices and the type of flying that they do. There is a trade off here between doing an analysis with very wide scope and doing an analysis with only the most certain data. We decided to use as much of the technical aircraft data as we could, even though this meant using less certain and extrapolated data for some of the explanatory variables.

We have data on wages in airlines and on the real interest rates in each country. The wage data is very patchy and noisy and we extrapolated a lot of values from the observations we had in the same way that, for example, the Penn World Table extrapolates from surveys. There are no taxes on aircraft fuel for international travel and the price of fuel reported by Platts does not vary a lot around the world. But countries can tax fuel for domestic aviation. We could only find data on these specific taxes for a small number of countries in a single year. So, we used proxies, such as the price of road gasoline and oil rents, for this variable. We proxy the type of flying airlines do using the characteristics of their home countries.

The most robust results from the analysis – that hold whether we use crude fuel economy or fuel economy adjusted for size and age – are that – all things constant – larger airlines select planes with higher fuel economy, higher interest rates are associated with poorer fuel economy, higher fuel prices are associated with higher fuel economy (but the elasticity is small), and fuel economy is worse in Europe and Central Asia than other regions.

It seems that for a given model age and size, more fuel efficient planes cost more. This would explain why, even holding age and size factors constant, higher interest rates are correlated with worse fuel economy. Also, if larger airlines have more access to finance or a lower cost of capital they will be able to afford the more fuel efficient planes.

What effect could carbon prices have on fleet fuel economy? The most relevant elasticity is the response of unadjusted fuel economy to the price of fuel. This allows airlines to adjust the size and model age of planes in response to an increase in the price of fuel. We estimate that this elasticity is -0.09 to -0.13, which suggests the effect won't be very big. Because we use proxies for the price of fuel, we expect that the true value of this elasticity is actually higher. The elasticity also assumes that there is a given set of available aircraft models. Induced innovation might result in more efficient models being developed. There might also be changes in the types of airlines and flights. So the effect could be quite a bit larger in the long run.


  1. Thanks for the reminder of times at NASA in the mid 70's. The Advanced Missions and Concepts Div. at NASA Ames was studying technologies to improve transport fuel efficiencies. We also studied hypersonic transports as well as dirigibles. Mach 12 to 60mph.
    One of my co-workers did a study to determine the best flight profile for max fuel efficiency. It was the same, almost, as for cars. Climb, coast, climb. I think for cars it is accelerate, coast accelerate. You might find his paper in NASA archives under Mark Ardema.

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